The present invention is directed to an OFDM/DMT digital communications system. More particularly, the present invention is directed to an apparatus and method for managing allocated bandwidth in a multi-point, OFDM/DMT digital communications system.
Multi-point communications systems having a primary site that is coupled for communication with a plurality of secondary sites are known. One such communications system type is a cable telephony system. Cable telephony systems transmit and receive telephone call communications over the same cable transmission media as used to receive cable television signals and other cable services.
One cable telephony system currently deployed and in commercial use is the Cablespan 2300 system available from Tellabs. Inc. The Cablespan 2300 system uses a head end unit that includes a primary transmitter and primary receiver disposed at a primary site. The head end unit transmits and receives. telephony data to and from a plurality of remote service units that are located at respective secondary sites. This communication scheme uses TDM QPSK modulation for the data communications and can accommodate approximately thirty phone calls within the 1.9 MHz bandwidth typically allocated for such communications.
As the number of cable telephony subscribers increases over time, the increased use will strain the limited bandwidth allocated to the cable telephony system. Generally stated, there are two potential solutions to this bandwidth allocation problem that may be used separately or in conjunction with one another. First, the bandwidth allocated to cable telephony communications may be increased. Second, the available bandwidth may be used more efficiently. It is often impractical to increase the bandwidth allocated to the cable telephony system given the competition between services for the total bandwidth available to the cable service provider. Therefore, it is preferable to use the allocated bandwidth in a more efficient manner. One way in which the assigned bandwidth may be used more efficiently is to use a modulation scheme that is capable of transmitting more information within a given bandwidth than the TDM QPSK modulation scheme presently employed.
The present inventors have recognized that OFDM/DMT modulation schemes may provide such an increase in transmitted information for a given bandwidth. Such systems, however, present a number of technical problems. One such problem is the determination of how the system is to handle the bandwidth allocated to it without disrupting the continuity of the communications that the system is handling. This problem may become acute in a point-to-multipoint cable telephony system where the sum of the bandwidth allocated to each of the remote units exceeds the total available bandwidth allocated to the overall system while, at the same time, optimizing for system performance. The present inventors have recognized the need to manage the allocated bandwidth of the overall system so as to minimize the likelihood of system bandwidth overload.
A multi-point communications system having an allocated total bandwidth is set forth. The system comprises a head end unit disposed at a primary site. The head end unit includes a receiver for receiving OFDM/DMT modulated data from a plurality of remote transceivers over a predetermined number of bins from a transmission medium. The receiver of the head end unit receives OFDM/DMT modulated data corresponding to a first data stream from a first one of the plurality of remote transceivers over one or more bins of a first subset of the predetermined number of bins and OFDM/DMT data corresponding to a second data stream from a second one of the plurality of remote transceivers over one or more bins of a second subset of the predetermined number of bins. The head end unit and each of the first and second transceivers can receive and/or transmit in at least a given X QAM mode or Y QAM mode, where X and Y designate constellation size use for QAM communications in a given bin. The head end unit monitors actual bandwidth usage of the total allocated bandwidth and uses the measured bandwidth usage to alter the QAM modes used for communications within the system.
Other features and advantages of the present invention will become apparent upon review of the following detailed description and accompanying drawings.